Vibrometer with hermetically sealed magnet capsule



Aug. 26, 1969 M4 T. ZIMMERMAN 3,453,945

VIBROMETER WITH HERMETICALLY SEALED MAGNET CAPSULE Filed March 20, 1968'United States Patent O 3,463,946 VIBROMETER WITH HERMETICALLY SEALEDMAGNET CAPSULE Michael Tone Zimmerman, West Covina, Calif., asslgnor,

by mesne assignments, to Bell & Howell Company,

Chicago, lll., a corporation of Illinois Filed Mar. 20, 1968, Ser. No.714,502 Int. Cl. H02k 33/16, 35/02 U.S. Cl. 310-15 8 'Claims ABSTRACT OFTHE DISCLOSURE An instrument vibrometer in which a magnet is mounted forreciprocation within a hermetically sealed capsule, the capsule beingremovably held within a coil, both the coil and the direction ofmovement of the magnet being aligned with a sensitive axis of thevibrometer within a vibrometer housing. The hermetic seal of the magnetcapsule assures that the bearings which mount the magnet within thecapsule for movement relative to the coil cannot become clogged byforeign matter entering the housing.

BACKGROUND OF THE INVENTION Field of the invention This inventionpertains to instrument vibrometers of the movable magnet, stationarycoil type. More particularly, it pertains to such vibrometers in whichthe magnet is mounted for movement within a hermetically sealed capsuleremovably mounted within the coil.

Review of the prior art A vibrometer is an instrument transducer whichdevelops an output signal the value of which is proportional to thevelocity of vibrations and displacements applied to the instrument.Vibrometer output signals may be integrated to determine the amount ofdisplacement produced by a particular vibration.

A common type of vibrometer, presently enjoying wide use, includes anelongate coil which is aligned with a sensitive axis of the vibrometerwithin a housing for the device. The coil conventionally is mounted inxed relation to the housing. A permanent magnet is spring-loaded withinthe housing for movement coaxially of the coil. The magnet acts as aseismic mass which tends to remain stationary as the housing and thecoil are moved in response to vibrations applied to the housing alongthe sensitive aXis at a frequency above the natural frequency of themagnet suspension. Relative movement between the coil and the magnetresults in the generation of an electrical signal within the coil, whichsignal is measured to determine the velocity of vibrations producingsuch relative movement.

The movable magnet conventionally is an elongate cylindrical elementwhich is disposed Within a bearing sleeve. The magnet itself, at each ofits opposite ends, carries a circumferential bearing ring, often made ofgold-palladium alloy. The bearing rings cooperate with the inner wallsof the bearing sleeve to provide low-friction, guiding support for themagnet. The bearing sleeve is disposed within a support spindle for thecoil.

Volume users of existing vibrometers can ill afford to spend the timeand money to return a vibrometer to the manufacturer each time thevibrometer fails for any reason. For this reason, in response to userdemands, it is standard practice to make such instruments eldrepairable. Field repairability in turn requires that the instrumenthousing be openable. Bolted and gasketed housing closures, however, asnoted above, breathe in use. Also, field repairs are not usually made bypersons having the "ice high level of skill of the manufacturer and arerarely made under the same conditions of cleanliness as the originalinstruments, with the result that foreign matter frequently is left inor enters the bearing sleeve during the lield repair operation. Thus,the principal cause of failure of existing magnet-and-coil lvibrometersis now recognized to follow directly from eld repairabilityspecifications imposed by the users of such instruments and from thefield repair procedures practiced by the users. The problem 0f foreignmatter contamination cannot realistically be overcome by hermeticallysealing the instrument housings because the users of the instrumentsinsist upon being able to repair such instruments themselves in thefield; also, it is desirable to have access to the interior of thehousing for the purposes of repairing failures not produced by foreignmatter contamination.

In view of the relative simplicity of their construction,magnet-and-coil vibrometers fail or require repair only for limitedreasons, but in existing vibrometers these few reasons produce a Veryhigh number of failures. Foreign matter may enter the vibrometer housingand cause clogging of the ring-sleeve bearings, in which case thebearings must be cleaned; the presence of foreign matter in the bearingsleeve is the greatest cause of vibrometer failure. After extended use,the bearing rings may become so abraded and worn that replacement of themagnet, with its bearing rings, and the bearing sleeve is required; thisis the next most frequent cause of vibrometer failure but isconsiderably less common than failure by reason of clogged bearings.Another cause of failure is separation of a conductor in the coil.

It should also be understood that in prior magnet-andcoil vibrometers,all the internal components of the vibrometer are disposed in a commonchamber within the housing, the chamber being closed in use of thevibrometer by a closure cap which is bolted and gasketed to the housing.

Magnet-and-coil vibrometers are extensively used to monitor enginevibration in commercial and military aircraft powered by jet or gasturbine engines. Such aircraft are operated at altitudes from zero to40,000 feet or more. In such applications, especially where thevibrometer is mounted close to the engine, the vibrometer must operatethrough a temperature range extending from sub-zero temperatures toapproximately 700 F., and through an ambient pressure range extendingfrom one standard atmosphere down to a minor fraction of a standardatmosphere. No matter how much care is exercised in gasketing thehousings of existing magnet-and-coil vibrometers, the housings of suchvibrometers breathe in use. Breathing occurs as air flows out of thehousing in going from low altitudes to high altitudes, and as air anddust enter the housing in going from high altitudes to low altitudes. Asa result, the bearings in such vibrometers rapidly become contaminatedwith foreign matter such that the magnets stick in their bearing sleeveand do not function to generate the desired signals. Even slightsticking of the magnet is a serious problem in vibrometers of themagnet-andcoil type.

SUMMARY OF THE INVENTION This invention provides a practical, effectiveand eflicient solution to the problem reviewed above. The presentvibrometer, of the magnet-and-coil type, complies with user imposedrequirements of field repairability yet uses hermetic seals tosignificantly reduce, if not altogether eliminate, the adverse effectsof foreign matter in the interface between the magnet bearing rings andthe bearing sleeve.

Generally speaking, the present invention provides a vibrometer whichincludes an openable housing. A coil is disposed in the housing inalignment with a sensitive axis of the vibrometer, the coil being xedrelative to the housing. A magnet is disposed in the housing for move- Yment along the sensitive axis within the coil in response to vibrationforces applied to the housing along the sensitive axis. In this context,the improvement provided by this invention comprises a hermeticallysealed containerdisposed within the coil and within which the magnet isdisposed for movement as aforesaid. Means are provided for removablymounting the container within and in fixed relation to the housing.

Thus, the moving parts of the vibrometer are located in a sealedcontainer where dust and dirt breathed into the housing cannot causeharm. Also, the housing is openable to permit access to the coil forfield repair thereof.

DESCRIPTION OF THE DRAWINGS The above-mentioned and other features ofthis invention are more fully set forth in the following detaileddescription of the invention, which description is presented withreference to the accompanying drawings, wherein:

FIG. 1 is a cross-sectional elevation view of a magnetand-coilvibrometer; and

FIG. 2 is an elevation view taken along line 2-2 of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT An instrument vibrometerincludes a housing 11 delined by a body 12 and a closure cap 13. Thebody delines .an internal axial bore 14. An external peripheral mountingange 15 is provided at the lower end of the body and an externalperipheral ange 16 is provided at the upper end of the body forcooperation with the closure cap. The lower end of bore 14 is sealed bya closure disk 17 which is welded to the -body as at 18. The uppersurface of the closure disk defines a short centering pin 19 whichextends along the axis of bore 14, the axis of bore 14 being theSensitive axis 20 of the vibrometer.

Closure cap 13 defines an internal chamber 21 which communicates withthe upper end of body bore 14 when the cap is secured to the body bybolts 22 threaded into body iiange 16. An O-ring gasket 23 provides aseparable seal ybetween the closure cap and flange 16 when the cap isbolted to the body.

An externally threaded connector component body 25 is hermeticallysealed to cap 13, as by weld 26. As shown in FIG. 2, a pair of terminalmembers 27 extend from adjacent the upper end of body bore 14 throughbody 25 to ends disposed within a recess 28 formed in body 25. Theterminal members are hermetically sealed to the connector body byceramic insulator sleeves 29.

An electrical coil 30 is wound around the exterior of an elongate coilspindle 31 disposed within the housing in axial alignment with sensitiveaxis 20. The coil spindle has an axial cylindrical -bore 32therethrough. Adjacent its lower end, the coil spindle denes adownwardly facing positioning shoulder 33 which is concentric to thebore 32 and which opens to an enlarged diameter internally threadedrecess 34 delined at the lower end of the coil spindle. An externallythreaded retainer ring 35 is engaged with the threads of recess 34. Theretainer ring detines an axial bore 36 having a diameter sized to make aclose tit with the diameter of centering pin 19.

The upper end of the coil spindle is permanently secured, as by weld 38,to an enlarged head 39 of a mounting stud 40 which has an externallythreaded, upwardly extending shank 41. Shank 41 passes through a hole 42which communicates cap chamber 21 with a recess 43 formed in the uppersurface of the cap concentric to sensitive axis 20. Stud 40 ismaintained coaxial with sensitive axis 20 by cooperation of apositioning shoulder 44 within a mating recess 45 formed in the cap atthe lower end of hole 42. The mounting stud is secured to the cap by anut 46 engaged with the shank thereof in recess 43. An O-ring gasket 47is engaged between the closure cap and stud head 39 to provide aseparable seal between the cap and the mounting stud. When the closurecap is bolted to the housing body, stud 40, retainer ring 35 andcentering pin 19 cooperate to position the coil spindle concentric tosensitive axis 20.

The opposite ends of the conductor which defines coil 3) are secured tocorresponding ones of terminal elements 27.

In view of the construction described above, it is apparent that whenbolts 22 are removed from cooperation with housing body liange 16 theclosure cap, the connector body and the coil spindle, including retainerring 35, are removable as a unit from the housing body.

An elongate cylindrical permanent magnet 50 is disposed within theinterior of the coil spindle for reciprocal movement along sensitiveaxis 20 in response to vibratory forces applied to housing 11 in thedirection of the sensitive axis. Adjacent each of its opposite ends, themagnet carries a circumferential bearing ring 51 which may be made ofgold-palladium alloy, especially where the vibrometer is to be subjectedto high temperature environments, the clearance shown in FIG. 1 betweenthe outer diameter of the bearing rings and the inner diameter ofbearing sleeve 53 has been exaggerated for the purposes of illustration.The magnet is mounted within a hermetically sealed container 52 dened byan elongate tubular bearing sleeve 53 and by upper and lower sleeveclosure plugs 54 and 55, respectively. The inner diameter of the bearingsleeve is sized to form a sliding lit with the outer diameter of bearingrings 51. The outer diameter of the bearing sleeve is sized to litwithin bore 32 of the coil spindle.

Upper sleeve closure plug 54 is hermetically sealed to the bearingsleeve as by welds 56. The lower end of the bearing sleeve defines anoutwardly extending peripheral flange 57, lower sleeve closure plug 55being secured to and hermetically sealed to the sleeve as by welds 5Smade around the outer diameter of flange 57. Flange 57 cooperates withspindle positioning shoulder 33 to position the magnet container in apredetermined relation to the coil during use of the vibrometer.Container liange 57 is clamped against positioning shoulder 33 byretainer ring 35.

A hole 60 is formed through lower container plug 55 from a recess 61which opens to the lower surface of the closure plug. Hole 60 isprovided for withdrawing air from the interior of the container and forreplacing the evacuated air with a dry inert gas; after the interior ofthe container has been charged with dry inert gas, hole 60 ishermetically sealed by resistance welding a valve ball 62 across theopening of hole 60 to recess 61.

A pair of coil springs 65 of selected stiifness are engaged one betweeneach end of the magnet and the adjacent sleeve closure plug. The springscooperate to yieldably bias the magnet into an at-rest position whichhas a predetermined relation to the length of coil 30. Preferably theat-rest position of the magnet is centrally of the length of the coil.

When vibratory forces are applied to vibrometer 10` in the direction ofsensitive axis 20, the magnet functions as a seismic mass in that ittends to remain stationary while the housing moves freely relative tothe magnet in response to the applied forces. Accordingly, the magnetmoves reciprocally within the housing along sensitive axis 20 relativeto coil 30. Such relative movement between the magnet and the coilgenerates an electrical signal within coil 30, which signal may bemonitored as a measure of the velocity of applied vibratory forces ormay be integrated to determine the extent the housing is displacedduring the production of the signal.

From the construction described above and illustrated in FIG. 1, it isapparant that magnet container 52 is readily removable from the coilspindle once the coil spindle has been removed from the housing body byremoving the housing closure cap from the housing body. The container isremovable from the coil spindle merely by disconnecting retainer ring 35from the lower end of the coil spindle.

Since magnet 50 of vibrometer 10 is located within a hermetically sealedcontainer, there is no possibility for foreign matter to enter theinterface between bearing rings 51 and bearing sleeve 53 during use ofthe vibrometer. Thus, the major cause of failure in prior vibrometers issignificantly reduced, if not altogether eliminated, in the presenttransducer. Also, since the magnet of the present vibrometer operates ina controlled atmosphere in the absence of foreign matter, the life ofbearing rings 5'1 is extended significantly over the life of bearingrings in prior devices; the presence of foreign matter on the bearingrings in prior devices accelerated the tendency of such bearing rings towear. Thus, as a practical matter, the only cause for the magnet tostick relative to the bearing sleeve would be produced by particlesabraded from the bearing rings themselves; when the bearing rings haveWorn to this extent, the magnet capsule is merely replaced. Thereplacement of the magnet capsule can be accomplished in the field byunskilled labor Working under adverse conditions without adverse effectupon the vibrometer itself.

From the foregoing description, it will be seen that this inventionmaintains the field repairability criterion demanded by users ofvibrometers of the type described. Also, since the magnet is containedWithin a hermetically sealed housing, the need for field repair of thevibrometer is substantially reduced relative to the frequency with whichfield repairs have had to be made to prior vibrometers. It isanticipated that field repair of the present vibrometer would berestricted to replacement of magnet capsules when the bearings thereinhave become worn, and to repair of broken connections within the coiland between the coil and the terminal elements. A user of the presentvibrometer need only maintain a small supply of spare magnet capsules onhand, and replaced capsules may be sent by the user to the manufacturerfor reworking. In essence, then, the present invention provides avibrometer which is essentially insensitive to the environment in whichit is used and to the vagaries of personnel charged with theresponsibility of field repair of such vibrometers.

What is claimed is:

1. A vibrometer comprising an openable housing, a coil disposedconcentric to a sensitive axis of the vibrometer, a magnet disposed inthe housing coaxially of the coil, means for mounting one of the coiland the magnet for movement relative to the housing along lthe sensitiveaxis and for mounting the other of the coil and the magnet stationaryrelative to the housing, a hermetically sealed container for the movableone of the coil land the magnet, and means removably mounting thecontainer within the housing in fixed relation thereto.

2. A vibrometer comprising `an openable housing, a coil aligned in thehousing with a sensitive axis of the vibrometer and fixed relative tothe housing, a magnet disposed in the housing for movement along thesensitive axis within the coil in response to vibration forces appliedto the housing along the sensitive axis, a hermetically sealed containerdisposed within the coil and within which the magnet is disposed formovement as aforesaid, and means removably mounting the container withinthe housing in fixed relation thereto.

3. A vibrometer according to claim 2 including means for biasing themagnet into a predetermined position relative to the coil.

4. A vibrometer according to claim 3 wherein the biasing means isdisposed within the container.

5. In a vibrometer having a sensitive axis and including an openablehousing, an elongate hollow coil spindle disposed in the housing infixed relation thereto concentric to the sensitive axis, a coil woundupon the coil spindle, and a magnet disposed within the coil spindle formovement along the sensitive axis in response to housing displacingforces applied to the housing along the sensitive axis, the improvementcomprising an elongate hermetically sealed -container within which themagnet is disposed for movement relative to the coil, the containerbeing disposed within the coil spindle so as to be stationary relativeto the housing in use of the vibrometer and to be removable as a Isealedunit vfrom the housing.

6. A vibrometer according to claim 5 wherein the magnet has a lengthbetween opposite ends thereof less than the length within the containerbetween the closed ends thereof, and a spring engaged between each endof the magnet and the adjacent end of the container for biasing themagnet into a predetermined position along the length of the container.

7. A vibrometer according to claim 5 including a dry inert -gas withinthe container.

8. A vibrometer according to claim l5 including bearing meanscooperating between the magnet and the container for guided movablesupport of the magnet in the contalner.

References Cited UNITED STATES PATENTS MILTON O. HIRSHFIELD, PrimaryExaminer D. F. DUGGAN, Assistant Examiner U.S. Cl. X.R.

